1. Field of the Invention
The present invention pertains to resistive potentiometer circuitry, and more particularly, it pertains to resistive potentiometer circuitry for generating a current linearly proportional to the displacement of a movable member.
2. Description of the Prior Art
Transducers for measuring pressure, temperature, or flow rates of fluids are often distributed at periodic intervals along a system of conduits and are arranged to be interrogated from a remote central location where the status of the overall system can be continuously monitored and analyzed. Because of the vast number of transducers required in many of such systems, emphasis in the design of such transducers and the circuitry therefor is placed upon simplicity, compactness and low cost of manufacture. The design of the transducers is further complicated by the existence of miles of wiring between the transducers and the central monitoring equipment, the large internal resistance inherent in the remotely located monitoring equipment, the need to minimize total current supplied to the transducer circuitry, and the need to conserve the number of wires dedicated to the transducers.
Gas flow and pressure transducers used by telephone companies provide a typical example of the aforementioned types of transducers and associated electrical circuitry. In order to prevent moisture damage to telephone cables, such cables are typically filled with dry air under pressure. In the event of a small break in the waterproof conduit surrounding the telephone wires, the pressurized dry air flows outwardly through the break to both prevent water penetration and to continually dry the broken surfaces. Transducers spaced along the length of the telephone cables continuously measure the pressure therein to insure the adequacy of the pressure to provide the appropriate protection to the telephone wires, and other transducers continuously measure the flow rate at various spaced positions along the cables in order to determine the location and severity of any breaks in the cable. Because of the long distances involved in a typical network of telephone cables, reasonable accuracy in the readings provided by the transducer outputs requires that the influence of the long interconnecting wire be minimized, that the currents drawn from the central monitoring equipment by the transducers be minimized, that minimum signals be provided in the absence of a stimulus at the transducer to permit the monitoring equipment to establish the integrity of the transducer circuitry connection, and that maximum output signals from the transducer circuitry be limited to avoid a condition that might be interpreted as a short. Such requirements have rendered it difficult to produce a simple, low cost potentiometer type circuit to provide the requisite current signals.
In order to meet their specific requirements, the telephone companies utilizing the aforedescribed transducer monitoring systems specify that the transducer circuitry present a resistance varying in accordance with the following equation: EQU R=(3.9-1.9X)/(1+19X)
where R is expressed in Megohms and X is the ratio of the applied stimulus (gas flow through the transducer or gauge pressure applied to the transducer) to the stimulus required to evoke a full scale response from the transducer. This equation can be separated into fixed and variable components to give the equation EQU R=0.1+r=0.1+3.8(1-X)/(1=19X)
where r is the variable component. From the foregoing, it will be apparent that small changes in stimulus (increased flow or increased pressure) produce very much larger changes in transducer resistance when the percentage of total movement is small then will be produced when the percentage of total movement is large. That is to say, with pressures or flow rates near zero, small increments of change produce greater changes in resistance than when the flow rates or pressures are near their maximum values. Thus, the requirement for a linear output current proportional to the flow rate or pressure has proven difficult to achieve with conventional off-the-shelf circuitry or with easily manufactured elements.
Another factor limiting the design of a resistive potentiometer element for the foregoing requirements is the desirability of point contact between the movable and fixed elements of the potentiometer in order to minimize frictional forces in the transducer and hence prevent inaccurate readings.
Because of the extreme differences in the responses required at the opposite ends of the scale, currently available continuously variable resistive potentiometer elements have not been found to be acceptable. Thus, telephone companies typically use stepped variable resistors to approximate the required response in constructing their variable resistor circuitry. In such arrangements, however, accuracy suffers due to the use of step changes to approximate separate sections of the response curve.